Intraband Polarization Research Articles

Dephasing and selection rules of interband and intraband polarizations in superlattices

We study the coherent dynamics of interband and intraband polarizations of Bloch oscillations in a GaAs/Al0.7Ga0.3 As semiconductor superlattice. Time-resolved measurements of both polarizations are simultaneously performed by combination of four-wave mixing and an electro-optic detection scheme in one experimental setup. This combination allows us to gain a much more comprehensive picture of the dynamics of a multilevel system. Distinctive differences in the oscillation frequencies and the dephasing of both polarizations are found. They arise from the fact that excitonic quantum beats are observed in four-wave mixing, whereas a quantum interference of electrons in continuum states is detected in the electro-optic signal. As a consequence, the polarizations are governed by different selection rules that explain the pronounced differences in the oscillation frequencies found for specific experimental conditions.

Ultrafast optical pump THz-probe spectroscopy on silicon

We investigate the dynamics of photoexcited carriers in silicon after interband excitation with optical femtosecond pulses by probing intraband transitions with time-delayed THz pulses. The experimental data at various temperatures are compared with numerical, self-consistent solutions of the intraband polarization and Maxwell’s equations. In addition, we deduce the carrier scattering rate from the comparison between experiment and theory.

Exceptionally slow dephasing of electronic continuum states in a semiconductor.

Fraunhofer-Institutfur Angewandte Festkorperphysik, D-79108Freiburg, Germany(Received 7 November 1994)The coherent excitation of excitonic and continuum Wannier-Starkstates in a semiconductorsuperlattice with femtosecond laser pulses leads to a modulation of both the interband and theintraband polarization. The field dependence of the Bloch oscillation frequency of the nonlinearinterband polarization is shown to differ strongly from that of the intraband polarization. Wefind that the observed modulation of the intraband polarization results from the superposition ofelectronic continuum states. This implies that the intraband dephasing time of these continuumstates is exceptionally longe

Bloch oscillations at room temperature.

We report on the observation of coherent electronic wave-packet oscillations in a semiconductor heterostructure at room temperature. Bloch oscillations are excited in a wide-miniband GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As superlattice by femtosecond laser pulses. Several cycles of the coherent electronic motion are observed within the dephasing time of the intraband polarization. The frequency can be tuned by the applied electric field from 4.5 to 8 THz.

The magnetic structure of SrFeO3 calculated within LDA

Abstract The magnetic properties of the antiferromagnetic perovskite oxide SrFeO3 are calculated using self-consistent ASW calculations based on the local density approximation (LDA). In an assumed non-magnetic configuration a large density of states of t2g-like iron states causes the instability towards intra-band polarization and the onset of the magnetic moment carried by Fe. The bonding features are discussed. The calculated values of the magnetic moments carried by Fe and O of 2.96 μB and 0.13 μB are within good agreement with experiment.

Electro-optic detection of Bloch oscillations.

The coherent motion of electronic wave packets in the Wannier-Stark regime of extemally biased GaAs/AIxGal _xAs superlattices is investigated by a time-resolved electro-optic technique. As predicted by the semiclassical theory, electronic wave packets undergo Bloch oscillations. They are observed via anisotropie changes in the refractive index associated with a intraband polarization, which is set up by the coherent spatial motion of electronic wave packets relative to localized holes. The oscillation amplitude depends strongly on the initial excitation conditions. Frequency, amplitude, and dephasing of Bloch oscillations are investigated over a wide range of electric fields. Complementary results to four-wave mixing and THz emission experiments are obtained.

Two-band density matrix theory: the effect of external fields on transition and transport processes

We present a derivation of the two-band density matrix dynamics which considers the effect of external fields on both the intraband drift motion in the six-dimensional configuration space and the interband transitions. This includes in particular the contribution of the light field to the drift term of the interband density matrix which is, for example, responsible for the 'resonant' Stark effect. A relation between the density matrix and electrodynamic quantities is established where our approach directly accounts for the decomposition into interband and intraband polarization. The consideration of a static magnetic field without invoking the 'Peierls substitution' turns out to yield deviations from the usual formulations in magneto-optics and transport theory. In the regime of quantum transport we obtain a general expression for the quantum correction of the energy density which is valid for nonequilibrium conditions and arbitrary band structure and can be used for the derivation of quantum hydrodynamic equations.

Non-equilibrium screening and plasmons in a coherently pumped semiconductor

The dielectric function of a coherently pumped semiconductor is calculated in the RPA to arbitrary order in the pump field. The theory starts with the longitudinal intraband polarization function. The diagonal and off-diagonal retarded one-particle Green's functions as well as the distribution functions are derived by means of the non-equilibrium Keldysh formalism. Exact analytical expressions follow within the collisionless regime and the rotating wave approximation. In this regime of the optical Stark effect the wavevector and frequency dependences of the resulting dielectric function are discussed for several values of the Rabi frequency and detuning between the pump frequency and energy gap. In the high-frequency limit the electron-hole pairs excited virtually exhibit the same Drude behaviour as real free carriers. For small frequencies and wavevectors there are remarkable discrepancies. The metallic properties of the virtual two-component plasma are depressed due to a finite transition energy. Moreover, contrary to what is found in the equilibrium case plasmons are strongly Landau damped already, even for vanishing wavevectors. Nevertheless, a screening function within the plasmon pole approximation is prepared.

Transient process in an injection laser with a non-quasi-Fermi electron distribution function

A calculation is made of the reaction of a semiconductor laser to rapid changes in the injection current. An allowance is made for deviations of the nonequilibrium carrier distribution function from the quasi-Fermi form when these deviations are characterized by an effective relaxation time τeff governed by the rates of intraband relaxation and polarization decay. It is shown that for typical parameters of a heterojunction laser the nature of its transient characteristic changes radically (from deep relaxation pulsations to a practically instantaneous repetition of the changes in the injection current) when τeff increases from 10−13 to 10−12 sec.

Microscopic considerations of long wavelength acoustic phonons in symmetry-induced zero gap semiconductors

The effective dynamical matrix for long wavelength acoustic phonons in a symmetry-induced zero-gap semiconductor is derived from microscopic considerations of the electron response to lattice displacement. A nonadiabatic theory is shown to be necessary in the very long wavelength limit for a pure system at T=0. Macroscopic correspondence is shown to require an effective charge-neutrality sum rule. This sum rule is used to examine acoustic phonons at somewhat shorter, but still long, wavelengths, and leads to the prediction of two regions of linear dispersion with different sound velocities separated in alpha -Sn at about a wavelength 103 cm. The degenerately doped system is also considered at low temperatures where it is shown that for wavenumbers much less than the Fermi wavenumber the acoustic phonons have a linear dispersion determined primarily by intraband polarization but with a change of character above kF.

Effect ofp32Intraband Polarization on the Mobility of Zero-Gap Semiconductors

It is shown that in doped zero-gap semiconductors like $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$, the intraband part of the static screening function is considerably decreased from that of free electrons at large momentum transfer by the $p$-like character of the conduction-band wave function. However, it is also shown that this large screening loss produces a relatively small change in the ionized-impurity-limited mobility.